Carbon monoxide adsorption for carbon monoxide clean-up in a fuel cell system

ABSTRACT

An apparatus removes carbon monoxide (CO) from a hydrogen-rich gas stream in a hydrogen fuel cell system. CO fouls costly catalytic particles in the membrane electrode assemblies of proton exchange membrane (PEM) fuel cells. A vessel houses a carbon monoxide adsorbent. The vessel may be a rotating pressure swing adsorber. A water gas shift reactor is upstream of the rotating pressure swing adsorber. The water gas shift reactor may include a second adsorbent adapted to adsorb carbon monoxide at low temperatures and to desorb carbon monoxide at high temperatures. The apparatus advantageously eliminates the use of a preferential oxidation (PROX) reactor, by providing an apparatus which incorporates CO adsorption in the place of the PROX reactor. This cleans up carbon monoxide without hydrogen consumption and the concomitant, undesirable excess low grade heat generation. The present invention reduces start-up duration, and improves overall fuel processor efficiency during normal operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/780,184 filed on Feb. 9, 2001. The disclosure of the aboveapplication is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to a fuel processor for ahydrogen fuel cell engine, and more specifically to such a processorwhich uses carbon monoxide (CO) adsorption for CO clean-up.

BACKGROUND OF THE INVENTION

In proton exchange membrane (PEM) fuel cells, hydrogen (H₂) is the anodereactant (i.e. fuel) and oxygen is the cathode reactant (i.e. oxidant).The oxygen can be either a pure form (O₂), or air (a mixture of O₂ andN₂). The solid polymer electrolytes are typically made from ion exchangeresins such as perfluoronated sulfonic acid. The anode/cathode typicallycomprises finely divided catalytic particles, which are often supportedon carbon particles, and mixed with a proton conductive resin. Thecatalytic particles are typically costly precious metal particles. Thesemembrane electrode assemblies are relatively expensive to manufactureand require certain conditions for effective operation. These conditionsinclude proper water management and humidification, and control ofcatalyst fouling constituents, such as carbon monoxide (CO).

For vehicular applications, it is desirable to use a liquid fuel such asalcohols (e.g. methanol or ethanol), other hydrocarbons (e.g. gasoline),and/or mixtures thereof (e.g. blends of ethanol/methanol and gasoline)as the source of hydrogen for the fuel cell. Such liquid fuels for thevehicle are easy to store onboard, and there is a nationwideinfrastructure for supplying liquid fuels. However, such fuels must bedissociated to release the hydrogen content thereof for fueling the fuelcell. The dissociation reaction is accomplished within a chemical fuelprocessor or reformer. The fuel processor contains one or more reactorswherein the fuel reacts with steam (and sometimes air) to yield areformate gas comprising primarily hydrogen and carbon dioxide. Forexample, in the steam methanol reformation process, methanol and water(as steam) are ideally reacted to generate hydrogen and carbon dioxide.In reality, carbon monoxide is also produced requiring additionalreaction processes. In a gasoline reformation process, steam, air andgasoline are reacted in a primary reactor which performs two reactions.One is a partial oxidation reaction, where air reacts with the fuelexothermally, and the other is a steam reforming reaction, where steamreacts with the fuel endothermically. The primary reactor produceshydrogen, carbon dioxide, carbon monoxide and water.

Reactors downstream of the primary reactor are required to lower the COconcentration in the hydrogen-rich reformate to levels tolerable in thefuel cell stack. Downstream reactors may include a water/gas shift (WGS)reactor and a preferential oxidizer (PROX) reactor. The PROX selectivelyoxidizes carbon monoxide in the presence of hydrogen to produce carbondioxide (CO₂), using oxygen from air as an oxidant. Here, control of airfeed is important to selectively oxidize CO to CO₂. Unfortunately, thepreferential oxidation reactor is not 100% selective and results inconsumption of hydrogen. The heat generated from the preferentialoxidation reactor is at a low temperature, resulting in excess low-gradeheat.

The operational gasoline fuel processor technologies to date do not meetautomotive targets for start-up durations, mass, and volume. Thestart-up time for such a system is limited by the time delay until thecombination of water gas shift and preferential oxidation reactors cansupply stack grade hydrogen. The start-up duration is related to themass of the catalyst system used for start-up and the energy needed toget the catalyst system up to its operating temperature. Anotherlimitation of the current technology is the inability to utilize the lowgrade heat such a system generates. Any heat loss reduces the fuelprocessor thermal efficiency.

Thus, it is an object of the present invention to provide a fuelprocessor for a hydrogen fuel cell engine which provides a means toreduce the carbon monoxide content under normal operation beforeentering the fuel cell stack, thereby advantageously eliminating the useof a preferential oxidizer (PROX) reactor. It is a further object of thepresent invention to provide such a fuel processor which provides quickcarbon monoxide uptake during start-up, thereby advantageouslyshortening start-up duration.

SUMMARY OF THE INVENTION

The present invention addresses and solves the above-mentioned problemsand meets the enumerated objectives and advantages, as well as othersnot enumerated, by providing an apparatus for removing carbon monoxide(CO) from a hydrogen-rich gas stream. In one aspect, the hydrogen-richstream is produced in a hydrogen fuel cell system which further includesmembrane electrode assemblies where such hydrogen is reacted with oxygento produce electricity. CO fouls costly catalytic particles in themembrane electrode assemblies, as described hereinabove. The apparatuscomprises a vessel housing an adsorbent adapted to adsorb the carbonmonoxide. The vessel may be a rotating pressure swing adsorber. Theapparatus further comprises a water gas shift reactor upstream of therotating pressure swing adsorber, wherein the water gas shift reactormay include a second adsorbent adapted to preferentially adsorb carbonmonoxide at low temperatures and to desorb carbon monoxide at hightemperatures.

The present invention advantageously eliminates the use of apreferential oxidation (PROX) reactor, by providing an apparatus whichincorporates CO adsorption in the place of the PROX reactor. The presentinvention provides a means to reduce carbon monoxide content whileminimizing hydrogen consumption and the concomitant, undesirable excesslow grade heat generation. The present invention reduces start-upduration, and improves overall fuel processor thermal efficiency duringnormal operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent by reference to the following detailed description anddrawings, in which:

FIG. 1 is a flow diagram of the fuel cell system of the presentinvention;

FIG. 2 is a flow diagram of an alternate embodiment of the fuel cellsystem of the present invention; and.

FIG. 3 is a partially schematic, perspective view of an exemplaryadsorber of the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Flow diagrams of a preferred and alternate embodiment for a fuelprocessor system for a fuel cell vehicle using an adsorber as theprimary means for carbon monoxide cleanup are shown in FIGS. 1 and 2,respectively.

Referring now to FIG. 1, hydrocarbon fuel such as, for example,gasoline, natural gas, methane, propane, methanol, ethanol, and/ormixtures thereof, etc. is fed into the fuel cell engine through stream11. The fuel is fed into primary reactor 1 where it reacts with thesteam/air mixture entering reactor 1 though stream 16. The steam isgenerated in heat exchanger 8, where liquid water from stream 13 isheated and vaporized by the hot exhaust stream 28 coming from combustor5. The steam exits heat exchanger 8 in stream 14 and is blended withcompressed air stream 17 in mixing valve 9. The steam/air mixture stream15 is further heated in heat exchanger 6 to form hot steam/air mixturestream 16 for feed into primary reactor 1. The heat required to raisethe temperature of stream 15 in heat exchanger 6 comes from stream 12,which is the effluent stream from primary reactor 1. Alternatively, theair and water can be heated separately and mixed either within or beforethe primary reactor 1.

Primary reactor 1 contains a steam reforming and/or partial oxidationcatalyst suitable for the specific fuel used. The temperature of reactor1 depends on the nature of the fuel and the relative compositions offuel, air and water, and is typically between about 300° C. and about1200° C. In primary reactor 1, the fuel is converted to a hydrogen-richreformate gas either by partial oxidation, steam reforming, orautothermal reforming. The reformate gas leaving primary reactor 1 instream 12 contains primarily hydrogen, nitrogen, carbon monoxide, carbondioxide, water, and possibly methane. The carbon monoxide concentrationin stream 12 is typically between about 1 mole % and about 20 mole %.Stream 12 is cooled in heat exchanger 6 as described above to theoperational temperature of water gas shift reactor 2. The cooledreformate gas exits heat exchanger 6 as stream 18. Alternatives (notshown in FIG. 1) allow for additional water to be fed directly intowater gas shift reactor 2 or blended with stream 18 as required by thewater gas shift reaction.

Water gas shift reactor 2 is either a high temperature water gas shiftreactor (320° C.-500° C.), a medium temperature shift reactor (250°C.-400° C.), or a low temperature water gas shift reactor (150° C.-250°C.). Alternatively, reactor 2 may consist of both a high and a lowtemperature water gas shift reactor with some means of cooling thereformate gas as it flows between the high and low temperature reactionzones. In reactor 2, carbon monoxide and water are converted to hydrogenand carbon dioxide via the water gas shift reaction.

One of the advantages of the present invention is the possibility ofusing only a high temperature water gas shift reactor, which isgenerally smaller than a low temperature water gas shift reactor, or asystem with both high and low temperature water gas shift reactors, eventhough the high temperature water gas shift reactor does not reduce theCO to very low levels due to equilibrium constraints. This is possiblebecause of the ability of the adsorber to handle relatively high COlevels that cannot be tolerated by conventional systems that usepreferential oxidation (PROX) reactors to convert CO to CO₂. Water gasshift reactor 2 may contain either high or low temperature water gasshift catalysts, or both, depending on the nature of the reactor asdescribed above. Conventional catalysts, such as Fe₃O₄/Cr₂O₃ for hightemperature shift or CuO/ZnO/Al₂O₃ for low temperature shift, may beused.

The product gas, stream 19, from water gas shift reactor 2 containsprimarily hydrogen, nitrogen, carbon monoxide, carbon dioxide and water.Stream 19 will typically contain about 0.5-5 mole % CO depending on thetemperature of reactor 2, the space velocity of the feed gas, the steamto carbon ratio, and the catalyst(s) used. Reformate stream 19 is cooledin heat exchanger 7 to the operating temperature of adsorber 3 (60°C.-100° C.) by the purge exhaust stream 24 from adsorber 3. Additionalcooling duty may optionally be provided by exchanging heat with cathodeexhaust stream 27 or water stream 13 to cool stream 20 to its requiredtemperature.

The cooled reformate stream 20 then flows to the high-pressure sectionof adsorber 3 where it is purified. The high-pressure section of theadsorber 3 will operate at a pressure between about 1.5 and about 5bar_(a). The detailed workings of adsorber 3 are described furtherhereinbelow.

There are several variations of the reformate product stream 21 leavingadsorber 3. In all the variations, CO levels in reformate stream 21 isless than about 100 ppm and preferably less than about 50 ppm. The COfrom stream 20 is adsorbed on the adsorbent(s) in adsorber 3. In oneaspect of the invention, only CO is adsorbed in adsorber 3 and reformateproduct stream 21 contains hydrogen, nitrogen, carbon dioxide and water.In a second aspect of the invention, both water and CO are adsorbed inadsorber 3, and reformate product stream 21 contains primarily hydrogen,nitrogen, and carbon dioxide.

In a third and more preferred aspect of the invention, nitrogen, carbonmonoxide, carbon dioxide and water are all adsorbed in adsorber 3, andreformate product 21 contains mostly hydrogen. In this aspect, thereformate product stream 21 contains at least about 99% H₂ andpreferably about 99.9% H₂, with the balance of stream 21 being nitrogen.Adsorbents that can be used for these three aspects are described below.

The H₂-rich reformate stream 21 from adsorber 3 is fed into the anodeside of fuel cell stack 4. Inside the stack 4, the hydrogen fromreformate stream 21 reacts with oxygen from air stream 26 to generateelectricity. Stream 26 is air at the operating temperature (ambient−100□C.) and pressure (2 and 5 bara) of fuel cell stack 4 and is fedinto the cathode side of the stack.

The fuel cell stack 4 generates exhaust gases from both the anode andcathode sides. The anode exhaust, stream 22, may contain hydrogen,nitrogen, carbon dioxide and/or water depending on the composition ofreformate stream 21. Anode exhaust stream 22 exits fuel cell stack 4 at,preferably, about 2-3 bar absolute pressure. Anode exhaust 22 isexpanded to atmospheric pressure in expander 10 (e.g. an isothermalexpander) to be used as a purge gas, stream 23, for desorption of theadsorbed gases from adsorber 3. Other means for reducing the pressure ofstream 22 can be employed, such as any variety of valves, nozzles,orifices, regulators, or the like. If an isothermal expander is used,the energy generated in expander 10, stream 30, can potentially serve todrive the rotor in adsorber 3 should a rotary adsorber be used (seebelow), although this is not essential for the function of the process.Expanded stream 23 should enter adsorber 3 at a temperature betweenabout 60□C. and about 100□C. and at about atmospheric pressure. Theadsorbed gases, which will include carbon monoxide, and possibly carbondioxide, nitrogen, and/or water, are desorbed using the principles ofpressure swing adsorption as described below. The purge gas exhauststream 24 contains the desorbed gases from adsorber 3 and is fed intoheat exchanger 7.

Another option not shown in FIG. 1 allows for reformate product stream21 to be split into two fractions before entering fuel cell stack 4,with the majority fraction of stream 21 still going to the stack and theremainder being expanded for use as the purge gas in adsorber 3. In thataspect of the invention, the anode exhaust 22 would be mixed withadsorber purge exhaust gas 24 and then fed directly to heat exchanger 7.

Alternatively, gas from the cathode exhaust, stream 27, which containsprimarily nitrogen, oxygen, and water, could be expanded and used as apurge gas for carbon monoxide desorption. This alternative is also notshown in FIG. 1. According to the invention, any gas stream orcombination of gas streams that does not contain a significant amount ofcarbon monoxide may be used as a purge gas for regeneration of thecarbon monoxide adsorbent. Other alternative purge gases include steam,air, nitrogen, or hydrogen, should they be available. The purge streamshould be at a lower pressure than the feed stream (stream 20) and atabout the same temperature as the feed stream. Another option allows forthe adsorber to be purged by using a vacuum pump to pull off theadsorbed gases rather than using a purge stream.

Adsorber exhaust gas 24 is heated in heat exchanger 7 by reformatestream 19. The heated exhaust gas 25 from heat exchanger 7 flows tocombustor 5. In this embodiment of the invention, cathode exhaust 27 isalso fed directly into combustor 5, although it may be heated beforeentering the combustor. Inside combustor 5, the hydrogen and carbonmonoxide from stream 25 react with the oxygen in stream 27 to formcarbon dioxide and water. Heat is generated by the exothermic combustionreactions and leaves combustor 5 as hot combustor exhaust stream 28.This heat is used to vaporize the water from stream 13 in heat exchanger8. Combustor exhaust stream 28 contains primarily nitrogen, carbondioxide and water. Stream 28 is cooled in heat exchanger 8, and exitsthe system as stream 29. Combustor exhaust stream 28 may also optionallybe used to preheat air stream 17. Liquid water may be collected fromstream 29 and used as water to be fed into the system in stream 13.

An alternate embodiment of the system of the present invention is shownin FIG. 2. There are two main differences between the systems shown inFIGS. 1 and 2. In the system shown in FIG. 2, 1) steam is used as thepreferred purge gas in adsorber 3 and 2) the desorbed CO is recycledback into the primary reactor. These differences are discussed below.

Referring now to FIG. 2, hydrocarbon fuel, air and steam are fed intoprimary reactor 1, as was the case in FIG. 1. Steam stream 14, which isgenerated by heating water stream 13 in heat exchanger 8, is fed intothe low-pressure section of adsorber 3 as a purge stream to desorb theadsorbed carbon monoxide. Superheated steam stream 14 is fed intoadsorber 3 at a temperature slightly above about 100□C. and at aboutatmospheric pressure. In this embodiment, the adsorbent in adsorber 3selectively adsorbs CO from stream 20 in the high-pressure section ofthe adsorber. Most of the hydrogen, nitrogen, carbon dioxide and waterin stream 20 passes through the adsorber and flows into the anode sideof fuel cell stack 4 in virtually CO-free reformate stream 21. Stream 21should contain less than about 100 ppm and preferably less than about 50ppm carbon monoxide.

Exhaust stream 33 from adsorber 3 contains the steam used aslow-pressure purge gas in the adsorber as well as the desorbed CO. Steamexhaust stream 33 is pressurized in compressor 10 to a pressure betweenabout 2 bar_(a) (bar absolute) and about 5 bar_(a). Alternativetechniques to pressurize the steam, such as an ejector, may be employed.Compressed steam stream 34 is mixed with compressed air stream 17 inmixing valve 9. The steam air mixture stream 15, which also contains thedesorbed CO from adsorber 3, is heated in heat exchanger 6 and fed toprimary reactor 1. An advantage of this embodiment is that the COadsorbed in adsorber 3 is recovered and recycled back into the system soit can be reacted with water in water gas shift reactor 2 to producemore hydrogen.

In an alternative to the embodiment of FIG. 2, compressed steam stream34, which contains the desorbed CO from adsorber 3, is split into twofractions. One fraction proceeds as described above, being mixed withair and sent to the primary reactor. The other fraction is either mixedwith reformate stream 18 or fed directly into water gas shift reactor 2.In another alternative, steam stream 14 is split into two fractions. Onefraction is blended directly with air stream 17 in mixing valve 9 to befed into primary reactor 1. The other fraction serves as thelow-pressure purge gas for desorption of CO from adsorber 3. The streampurge gas exhaust from adsorber 3, which contains the desorbed CO, isthen compressed and recycled back into water gas shift reactor 2, eitherdirectly or by mixing with reformate stream 18. Additional alternativesnot shown in FIG. 2 allow for additional water to be fed directly intowater gas shift reactor 2 or blended with stream 18 as required by thewater gas shift reaction.

Anode exhaust stream 22 is fed to heat exchanger 7 where it is heatedusing the reformate stream 19 after exiting water gas shift reactor 2.The hot anode exhaust stream 31 leaving heat exchanger 7 flows directlyto combustor 5 where it reacts with cathode exhaust stream 27 asdescribed in the embodiment of FIG. 1.

Gas streams besides steam may be used to purge the adsorber of CO as inFIG. 2. According to the invention, any gas stream or combination of gasstreams that does not contain significant amounts of carbon monoxide andcarbon dioxide may be used as a purge gas for regeneration of the carbonmonoxide adsorbent. Other alternative purge gases include cathodeexhaust stream 27, air, nitrogen, or hydrogen, should they be available.Another option allows for the adsorber to be purged by using a vacuumpump to pull off the adsorbed gases rather than using a purge stream.The purge stream should be at a lower pressure than the feed stream(stream 20) and at about the same temperature as the feed stream.

A further advantage of the present invention is the ability of thesystem to adsorb CO during startup, when the primary reactor isproducing hydrogen and CO, but the water gas shift reactor 2 is stillbelow its operating temperature. The incorporation of a CO adsorberpermits stack grade H₂ (<100 ppm CO) to be generated almost immediately.The startup scenario is similar in both of the embodiments of FIGS. 1and 2.

At startup, mixing valve 9 is set so that only air, and not water,enters the primary reactor along with the fuel until enough heat hasbeen generated in the combustor to produce steam. Hydrogen and carbonmonoxide are formed thermally or catalytically using a highly activecatalyst on a electrically heated support in primary reactor 1 and flowinto water gas shift reactor 2, which is still below its operatingtemperature and thus unable to shift CO and water into CO₂ and hydrogen.

At this point, one of two things may happen, depending on the exactnature of the system. If a low temperature CO adsorbent is added into orbefore water gas shift reactor 2 as described below, the CO in the coldreformate will adsorb on the adsorbent in water gas shift reactor 2. Aswater gas shift reactor 2 comes up to its operating temperature, the COwill desorb from the CO adsorbent and will be shifted to CO₂ on thewater gas shift catalyst(s). At that point, normal operation willcommence. If there is no CO adsorbent either inside or before water gasshift reactor 2, or there is not enough adsorbent to adsorb all of theCO in reformate stream 18, then the CO will pass through water gas shiftreactor 2 and continue via streams 19 and 20 into adsorber 3. Theremaining CO in stream 20 will adsorb in the high-pressure section ofadsorber 3. In general, adsorbers operate more effectively at lowertemperatures, so adsorber 3 will be able to adsorb CO effectively evenbefore it reaches its final operating temperature. In both cases, normaloperation can begin when 1) water gas shift reactor 2 has reached itsoperating temperature; and 2) enough heat has been generated incombustor 5 to produce the steam required for normal operation ofreactors 1 and 2.

The present invention is not intended to be limited to the specifics ofthe systems as shown in the FIGS. as many changes and variations totheses embodiments may be made without departing from the inventiveconcept. For example, additional heating and cooling of any streamoutside of the three heat exchangers described may be easilyaccomplished. Also, water may readily be recovered from a variety ofstreams, including streams 20, 21, 22, 24, 27, and 29 for reintroductioninto the system.

In the embodiment of FIG. 1, where the reformate product from adsorber3, stream 21, contains mostly hydrogen, adsorbent(s) should be usedwhich will adsorb carbon monoxide, carbon dioxide, water, and nitrogen.This could be done using a single adsorbent material such as a zeolite.Commercial zeolite adsorbents such as type 5A and type 13X, and mixturesthereof are known to adsorb all of these materials. However, thecapacity of these zeolites for carbon monoxide is relatively lowcompared to those of the other adsorbate gases. Therefore, it ispreferred to add an adsorbent that has a relatively high capacity forcarbon monoxide. For example, the carbon monoxide adsorbent may be ametal oxide or metal salt, such as copper, silver, or tin salt or oxideimpregnated or exchanged on activated carbon, alumina or zeolites, andmixtures thereof. See, for example U.S. Pat. No. 4,917,711 issued to Xieet al; U.S. Pat. No. 4,696,682 issued to Hirai et al.; U.S. Pat. No.4,587,114 issued to Hirai et al.; and U.S. Pat. No. 5,529,763 issued toPeng et al, each of the disclosures of which is incorporated herein byreference in its entirety. These carbon monoxide adsorbents mayselectively adsorb water instead of carbon monoxide. Therefore, a layerof a desiccant (water adsorbent) may be put in the adsorber vesselupstream of the carbon monoxide adsorbent. The desiccant may be anyconventional water adsorbent such as a zeolite molecular sieve,activated alumina, silica gel, or mixtures thereof.

In the embodiment of FIG. 2, where primarily CO is adsorbed in adsorber3, an adsorbent should be used which selectively adsorbs carbon monoxideover carbon dioxide in the presence of water and nitrogen. Such anadsorbent may be a metal oxide or metal salt, such as copper, silver, ortin salt or oxide impregnated or exchanged on activated carbon, aluminaor a high-silica zeolite such as type Y or ZSM-5.

The combining of materials can be accomplished by preparing layers ofdistinct particles, such as beads or extrudates, of the variousadsorbents. The adsorbent materials may also be preferably formedtogether into a single particle, such as a bead or an extrudate, or,most preferably, formulated into monoliths, foams, honeycombs or thelike.

Additionally, the water gas shift reactor may also contain a carbonmonoxide adsorbent that will adsorb carbon monoxide at temperaturesbelow the activation temperature of the water gas shift catalyst. Thiswill help to ensure that carbon monoxide will not break through thereactor during startup from ambient temperatures, when the water gasshift catalyst is still relatively cold, until the operating temperatureof the reactor is achieved. The carbon monoxide adsorbent will have amuch lower capacity for carbon monoxide at the higher steady-stateoperating temperature of the reactor. A layer of the carbon monoxideadsorbent may be placed in front of the water gas shift catalyst, or thetwo materials may be combined into a single layer. The carbon monoxideadsorbent may be a tin, copper or silver salt impregnated on activatedcarbon, alumina or a zeolite. The carbon monoxide is regenerated fromthe adsorbent as the reactor is heated to its operating temperature andcan then be either converted to carbon dioxide in the water gas shiftreactor or readsorbed downstream in the adsorber.

Conventional pressure swing adsorption (PSA) systems are very large andconsist of a minimum of two separate adsorption vessels complete withnumerous valves and manifolds. In a two-vessel system, one vessel wouldbe in the adsorption mode and the second vessel would be in variousstages of depressurization or blowdown, purge, and pressurization. Manycommercial hydrogen PSA cycles use four beds, with one bed in theproduction stage at any given time, and the other three beds in variousstages of equalization, blowdown, purge, and pressurization. See, forexample U.S. Pat. No. 3,453,418 issued to Wagner; and U.S. Pat. No.3,564,816 issued to Batta, each of the disclosures of which isincorporated herein by reference in its entirety. Also, some commercialhydrogen PSA cycles use twelve beds, with four beds in the productionstage at any given time, and the other eight beds in various stages ofequalization, blowdown, purge, and pressurization. See for example U.S.Pat. No. 3,846,849 issued to Fuderer et al., the disclosure of which isincorporated herein by reference in its entirety. These PSA cycle stagesare described in detail below. It is well known that PSA systems withmore than two vessels exhibit higher hydrogen recoveries and reducedpower by incorporating pressure equalization steps. These multiple,staged fixed bed PSA systems, however, contain complex valvearrangements and are non-continuous due to the cycling of these valves.

Alternatively, rotating adsorber vessels allow for continuous productionin a relatively small system with minimum valving. Rotating pressureswing adsorption systems are described by Petit et al in U.S. Pat. No.5,441,559; and by Keefer et al. in PCT publication No. WO 99/28013, eachof the disclosures of which is incorporated herein by reference in itsentirety. In order for the adsorber of this invention to be small enoughto fit in a vehicle, this invention preferably uses a single rotatingvessel 3 with only two fixed valve faces. Rotation of the vessel 3allows the adsorbent mixture to cycle between fixed regions foradsorption, depressurization, purge, and pressurization (as describedbelow) with cycle times much smaller than those of conventional PSAsystems. Further features of the rotary adsorber are described belowwith reference to FIG. 3. The cycle in which the adsorber is used willnow be described.

The cycle stages for the adsorber are as follows.

Adsorption

During the adsorption step, the reformate effluent from the water gasshift reactor 2 is fed over the adsorbent(s) at the higher feedpressure. In the embodiment of FIG. 2, only carbon monoxide adsorbs onthe adsorbent. Alternatively, in the embodiment of FIG. 1, carbonmonoxide, carbon dioxide, water, and nitrogen may all adsorb so theproduct gas contains greater than 99% hydrogen by volume. In that case,the remainder of the adsorber product is primarily nitrogen. In eithercase, the adsorber product gas will contain less than about 100 ppmcarbon monoxide, and, preferably, less than about 50 ppm carbonmonoxide. The production step is stopped before carbon monoxide breaksthrough the outlet of the adsorber vessel. At the end of the productionstep, the adsorbent is nearly saturated with the adsorbed gases and thevessel is at elevated pressure with hydrogen, carbon monoxide, carbondioxide, water, and nitrogen.

Depressurization

The adsorber vessel is depressurized from the feed pressure to the purgepressure by exhausting the gas in the direction counter-current to theadsorption direction. During depressurization, the outlet of theadsorber vessel is sealed. Alternatively, the vessel can bedepressurized co-currently, and the vessel inlet is sealed. Thedepressurization exhaust gas contains hydrogen, carbon monoxide, carbondioxide, water, and nitrogen. The exhaust will exit the adsorber vesselat atmospheric pressure and can be sent to the combustor or recycledinto another part of the adsorber or the fuel processor system.

Purge

The adsorber vessel is purged with the expanded exhaust from the PEMfuel cell stack, low pressure superheated steam, or other suitable purgegas as described above for the preferred embodiments of the invention(such as a fraction of the CO-free reformate). The purge stream flows atambient pressure in the direction counter-current to the adsorptiondirection. The adsorber may also be purged by pulling a vacuum from thedirection counter-current to the adsorption direction using a vacuumpump. The exhausted purge gas will contain most of the adsorbed carbonmonoxide and other adsorbed gases. The purge step is terminated whenessentially all of the carbon monoxide and other adsorbed gases havebeen desorbed from the adsorbent(s).

Pressurization

The adsorber vessel is pressurized back up to the adsorption pressure inthe same direction as the adsorption step using the cooled hydrogen-richproduct from water gas shift reactor 2, stream 20. Duringpressurization, the outlet of the vessel is sealed. Alternatively, thevessel may be pressurized using a fraction of the hydrogen-rich productfrom the adsorber, stream 21, in a direction counter-current to theproduction direction, and the vessel inlet is sealed. Afterpressurization, the adsorber returns to the adsorption step and thecycle continues indefinitely.

Equalization stages, which are well known to those skilled in the art ofPSA systems, may be added to the adsorption cycle to enhance hydrogenrecovery. For example, one vessel or section of the rotating vessel thathas just completed the adsorption step may be equalized, or connectedvia the outlets of both sections, with another section that has justcompleted the purge step. During this equalization, the pressure in thefirst section is reduced and the pressure in the second section isincreased accordingly. Also, the hydrogen remaining in the first sectionof the vessel at the end of adsorption is partially recovered in thesecond section, which has completed the purge step.

An exemplary rotary adsorber is shown in FIG. 3.

Description of Rotary Adsorber

A simplified schematic of an exemplary rotary adsorption apparatus isshown in FIG. 3. The rotary adsorber comprises a wheel 41 of adsorbentmaterial; an upper valve face 42; and a lower valve face 43. Theadsorbent wheel 41 is simply referred to as wheel 41 hereinafter. Uppervalve face 42 will be in direct contact with the top of wheel 41 andlower valve face 43 will be in direct contact with the bottom of wheel41. The wheel and the two valve faces form an assembly that is enclosedin a housing (not shown) in FIG. 3.

Wheel 41 is made up of pie shaped compartments 50. Preferably, there area minimum of twelve such adsorbent compartments. The compartments 50 areseparated from each other by walls 55. The walls 55 preventintercompartmental (tangential) flow, thus ensuring that the gases flowonly in the axial direction through wheel 41. Wheel 41 rotates in thecounterclockwise direction as indicated by arrow 51. Wheel 41 is rotatedeither by a rotor (not shown) that passes through the center of thewheel or a belt (not shown) in contact with the outer housing of thewheel. Stationary valve faces 42 and 43 do not move as wheel 41 rotates.

Upper valve face 42 is divided into subsections 64, 65, 66, and 67.These subsections are open windows that allow gas to flow through.Subsections 64, 65, 66, and 67 are separated by barrier seals 74.Barrier seals 74 prevent gas from flowing between the subsections.Stream 116 is the feed gas for the pressurization step of the PSA cycleand flows through subsection 64. Stream 117 is the feed gas for theproduction step of the PSA cycle and flows through subsection 65. Stream118 is the exhaust gas from the depressurization step of the PSA cycleand flows through subsection 66. Stream 119 is the exhaust gas from thepurge step of the PSA cycle and flows through subsection 67.

Lower valve face 43 is divided into subsections 72, 73, 78, and 79.Subsections 78 and 79 are open windows that allow gas to flow through.Subsections 72 and 73 are solid faces that prevent gases from flowingthrough. Subsections 72, 73, 78, and 79 are separated by barrier seals74. Barrier seals 74 prevent gas from flowing between the subsections.Stream 120 is the product gas from the production step of the PSA cycleand flows through subsection 79. Stream 121 is the feed gas for thepurge step of the PSA cycle and flows through subsection 78. Solid faceof subsection 73 prevents gas from entering or exiting the bottom ofwheel 41 during the pressurization step of the PSA cycle. Solid face ofsubsection 72 prevents gas from entering or exiting the bottom of wheel41 during the depressurization step of the PSA cycle.

The rotary adsorber system shown in FIG. 3 is a basic, generic deviceshown here for the purpose of identifying the key features of a rotaryadsorber and their functions. There are many variations to the devicenot shown in FIG. 3 that would provide a suitable device for use in thefuel processing system of the invention.

In one aspect, stream 116 of FIG. 3 is constituted by a part of stream20 of FIGS. 1 and 2. In another aspect, stream 117 of FIG. 3 isessentially the same as stream 20 of FIGS. 1 and 2. Similarly, in oneaspect, stream 118 of FIG. 3 is constituted by a part of stream 24 ofFIG. 1, and stream 118 is constituted by a part of stream 33 of FIG. 2.As shown here, stream 119 for exhaust gas purge is essentially the sameas stream 24 of FIG. 1, and stream 119 exhaust gas purge of FIG. 3 isessentially the same as stream 33 of FIG. 2. Finally, stream 120 of FIG.3 is essentially the same as stream 21 of FIGS. 1 and 2; and stream 121of FIG. 3 is essentially the same as stream 23 of FIG. 1, and stream 14of FIG. 2.

The pressurization and depressurization steps require a relatively smallflow stream and, therefore, as illustrated here, a portion of thestreams recited earlier with respect to FIG. 3, namely stream 116 andstream 118, respectively, are used. However, other methods could be usedto accomplish the pressurization and depressurization steps. Thus,streams 116 and 118 are relevant to the overall process in that theyfacilitate operation of the rotary adsorber system. The key featuresdescribed here pertain to processing of a feed stream designated asstream 20 of FIGS. 1 and 2 and stream 117 of FIG. 3, to provide aproduct stream designated as stream 21 of FIGS. 1 and 2 and stream 120of FIG. 3 to supply the anode.

Advantages of a fuel cell system using an adsorber for hydrogenpurification for automotive fuel cell applications as compared toconventional methods include one or more of the following:

-   -   1) The ability to remove CO at startup, therefore reducing the        time required to produce stack grade hydrogen;    -   2) The potential to produce 99.9% hydrogen (embodiment of FIG.        1);    -   3) The potential to recover CO for improved system efficiency        (embodiment of FIG. 2);    -   4) Elimination of low temperature water gas shift reactor, if        desired;    -   5) Elimination of precious metal catalysts typically used in        preferential oxidation (PROX);    -   6) No dilution of hydrogen product with nitrogen from air used        for preferential carbon monoxide oxidation;    -   7) Elimination of air compression for preferential oxidation        resulting in power savings;    -   8) Elimination of low-grade excess heat generated by        preferential oxidation; and    -   9) The ability to control the adsorption unit's cycling rate to        optimize efficiency over a wide range of operating loads.

While preferred embodiments, forms and arrangements of parts of theinvention have been described in detail, it will be apparent to thoseskilled in the art that the disclosed embodiments may be modified.Therefore, the foregoing description is to be considered exemplaryrather than limiting, and the true scope of the invention is thatdefined in the following claims.

1. A method for improving the quality of a hydrogen-containing gasstream comprising: producing a first gas stream having a hydrogencontent and a carbon monoxide content in a first reactor; passing saidfirst gas stream through a shift reactor to produce an effluent stream,said shift reactor including a first reaction region having a water gasshift catalyst and a second reaction region having a first carbonmonoxide adsorbent; adsorbing said carbon monoxide content from saidfirst gas stream in said second reaction region when said shift reactoris at a substantially ambient temperature and pressure condition; anddesorbing said carbon monoxide content from said first gas stream insaid second reaction region when said shift reactor is at a normal shiftreactor operating temperature and pressure condition which is above saidsubstantially ambient temperature and pressure condition.
 2. The methodof claim 1 further comprising purging said shift reactor by introducinga purge gas in a direction counter-current to said first gas stream. 3.The method of claim 1 further comprising passing said effluent streamthrough a first region of a second carbon monoxide adsorbent in apressure swing adsorber vessel to adsorb carbon monoxide from saideffluent stream.
 4. The method of claim 3 further comprising passing apurge gas through a second region of said second carbon monoxideadsorbent in said pressure swing adsorber vessel to desorb carbonmonoxide from said carbon monoxide adsorbent.
 5. The method of claim 4further comprising rotating said second carbon monoxide adsorbentthrough an adsorption region, a depressurization region, a purge regionand a pressurization region.